1. Field of the Invention
The present invention relates to a drive force control method for a four-wheel drive vehicle.
2. Description of the Related Art
In the case of turning a corner having a small turning radius in a four-wheel drive mode of a four-wheel drive vehicle in a low to medium vehicle speed range, a difference in rotational speed due to a difference in turning radius is generated between the front and rear wheels of the vehicle, causing a tight corner braking phenomenon. As the prior art for eliminating such a tight corner braking phenomenon, front and rear wheels driving devices are disclosed in Japanese Patent Publication Nos. 7-61779 and 7-64219.
The front and rear wheels driving devices disclosed in these publications have such a structure that a speed increasing device is provided between main drive wheels and auxiliary drive wheels to thereby adjust an average rotational speed of the auxiliary drive wheels to an average rotational speed of the main drive wheels. This speed increasing device includes a lockup clutch and a speed increasing clutch, which are selectively switched between ON and OFF states to thereby obtain a lockup condition or same speed condition where the average rotational speed of the main drive wheels and the average rotational speed of the auxiliary drive wheels are substantially equal to each other or an increasing speed condition where the average rotational speed of the auxiliary drive wheels is greater than the average rotational speed of the main drive wheels.
Particularly in the front and rear wheels driving device disclosed in Japanese Patent Publication No. 7-61779, a torque distribution ratio between right and left rear wheels are controlled according to a vehicle speed and a steering angle so that the rear wheel torque is larger than the front wheel torque and the turning outer wheel torque is larger than the turning inner wheel torque. In this front and rear wheels driving device, the auxiliary drive wheels are increased in rotational speed by the speed increasing device in turning a corner having a small turning radius in the four-wheel drive mode, thereby preventing the tight corner braking phenomenon.
The object of distributing a drive force between right and left wheels in turning is to give a larger drive force to the outer wheel as compared with the inner wheel, thereby generating a turning moment to suppress understeer occurring at acceleration during turning. However, the expectable effect of the drive force distribution between right and left wheels may vary according to a vehicle speed. For example, there is a tendency to desire stabilization of a vehicle body rather than improvement in maneuverability by giving a large turning moment at high vehicle speeds. However, minute control of the drive force distribution between right and left wheels according to a vehicle speed is not sufficiently disclosed in the above publications.
By distributing a driving/braking force generated by an engine in a four-wheel drive vehicle between front wheels and rear wheels, the load on each wheel can be controlled to maximize the utilization of tire performance. Particularly at acceleration of a two-wheel drive vehicle, a drive force so large as to cause skid of the wheels is generated. It is therefore very effective to make the two-wheel drive vehicle into a four-wheel drive vehicle.
In also taking a turning performance into consideration, the performance can be improved by changing the drive force distribution ratio between front and rear wheels so that the drive force distributed to the rear wheels is greater than that to the front wheels. However, when a vehicle speed is increased, a drive force and an engine brake force generated by a vehicle body are decreased in general, so that the effect of changing the drive force distribution ratio between front and rear wheels is small. Changing the torque distribution ratio of rear wheels to front wheels according to a vehicle speed, accelerator opening, transmission shift position, etc. is not sufficiently described in the above publications.